Mechanisms responsible for the diversity of plant defenses in natural populations are poorly understood. This dissertation examines how environmental heterogeneity and succession influence the distribution, expression, and adaptive value of resistance and tolerance of goldenrod (Solidago altissima). Within replicate early- and late-successional fields, I quantified the spatial structure of herbivore damage and goldenrod resistance, tolerance, and fitness traits, and assessed whether the spatial structure of defense was correlated with edaphic conditions and neighboring plant community. I then conducted a common garden experiment to empirically measure genetic variability of resistance and tolerance and quantify their associated fitness costs and selection gradients. Lastly, I conducted a field experiment to test the prediction that successional changes in the environment alter the adaptive value of resistance and tolerance. To this end, I transplanted goldenrod genets into early- and late-successional fields and measured defense levels, fitness costs, and selection gradients.

Field surveys revealed that defense traits were strongly spatially structured (i.e., autocorrelated), and late stages were more strongly structured than early stages. There were stage-specific relationships between defense and neighbor variables. Damage was correlated with vegetative cover in early stages, and tolerance traits were positively correlated with canopy cover in late stages. In the common garden, I found genetic variability in resistance and tolerance, but low heritability. Herbivores imposed strong selection for increased tolerance but not resistance. Results from the field experiment revealed an apparent succession-induced shift in the expression and adaptive value of resistance and tolerance. Resistance was 41% greater and tolerance was 97% lower for goldenrods planted in early than in late stages. Resistance was more adaptive in early stages and tolerance may be beneficial in mitigating the effects of both herbivory and low light availability in late stages.

This represents the first study to examine plant resistance and tolerance to herbivory in a spatiotemporal context. I conclude that environmental heterogeneity and succession may be important mechanisms promoting the spatial and temporal variability of plant defense expression in nature. Fluctuating adaptive landscapes may have important implications for trait evolution and should be incorporated into future studies of plant defense.